1. Field of the Invention
The present invention relates to a method and apparatus for detecting a disturbance along a length of optical fiber. More specifically, the present invention uses Polarization Optical Time Domain Reflectometry (POTDR) to ascertain if there has been a disturbance along the length of the optical fiber. This technique can be used for security applications in areas such as, but not limited to, fiber optic perimeter security, telecommunications, fire detection, and pipelines.
2. Description of Prior Art
A number of intrusion detection systems are commercially produced. Many of these systems have detection zone lengths from tens of meters up to several kilometers. Oftentimes, these systems will only identify merely whether or not an intrusion has occurred within the detection zone. It would be advantageous with an intrusion detection system to obtain the specific location of the disturbance within the detection zone. Pinpointing such location would enable quicker assessment of the intrusion via either video or human deployment.
Prior art in the field of fiber optic security equipment currently includes polarmetric multimode fiber optic sensors that rely on the differential coupling of light between polarisation states within a multimode optical fiber. When a disturbance occurs along the length of a multimode optical fiber, coupling between both the spatial modes propagating within the fiber and the polarisation eigenstates occur. Prior art fiber optic sensors use a multimode continuous wave laser diode. The system is operated in transmission. FIG. 1 shows a schematic diagram of the relevant optical parts of a fiber optic sensor such as that marketed as IntelliFIBER™ by Senstar-Stellar Corporation of Carp, Ontario, Canada. Polarized light is launched by a pigtailed laser diode 102 into a multimode sensor fiber 103. When the fiber 103 is disturbed, light is coupled between the s- and p-polarisation states. The frequency and strength of the coupling is dependent upon the frequency and strength of the disturbance. The s- and p-polarisation states are defined by the orientation of the plane-of-incidence of the polarisation beam splitter (PBS) cube 105. Transmitted light is emitted from the fiber 103 at a collimator 104 and into the s- and p-polarisation exit ports of the PBS cube 105. Light from the PBS cube 105 is then detected on pin silicon (Si) photodiodes illustrated by p-state detector 101 and s-state detector 106. The difference in the output voltages of the pin Si photodiodes 101, 106 is dependent upon the disturbance such that the difference is processed to identify an intrusion.
Other prior art fiber optic sensors use the redistribution of the energy in the spatial modes on a multimode fiber to detect a disturbance to the fiber. Examples of such include U.S. Pat. No. 5,144,689 issued to Lovely on Sep. 1, 1992 and PCT Publication WO9608695 filed by Tapanes on May 28, 1997. The optical configuration of such prior art sensors is similar to that shown in FIG. 1 except that the PBS is replaced with a modal filter (i.e., aperture). In the case of the prior art of U.S. Pat. No. 5,144,689, the fiber optic sensor includes an inline spacer used between the sensor fiber and a receiver fiber and in the case of PCT Publication WO9608695 a fiber with a smaller core diameter (i.e., single mode fiber) is fusion spliced onto the multimode sensor fiber. Both of these techniques result in a large loss of optical power.
The current prior art sensors as briefly described above are all equally unable to identify the location of disturbances within the detection zone, especially at any specific location along the length of the fiber.
The present invention therefore seeks to provide a fiber optic sensor mechanism that can effectively determine the location of disturbances along a length of optical fiber in a variety of security applications.